1. Technical Field
The invention relates generally to disk drive assemblies and, more particularly to frequency modulation read-back signal generation patterns for generating position error signals in accordance therewith for disk drive assemblies.
2. Description of Related Art
A magnetic disk drive assembly is a machine that is typically used to read data from and write data onto a magnetic disk. FIG. 1 illustrates a conventional magnetic disk drive assembly 32. The assembly 32 includes a storage medium 12 (e.g., a disk), an actuator 38, a head assembly 18 and a voice coil motor 40. The head assembly 18 is attached to the actuator 38 that is connected to the voice coil motor 40 (e.g., a servo motor). The voice coil motor 40 is used to move the head assembly 18 in order to keep it over a desired portion of the storage medium 12.
Information is recorded on the storage medium 12 in one or more tracks 34 and in one or more servo sectors 36. Typically, the tracks 34 store data, and for the disk drive assembly 32 to work properly, the head assembly 18 must lie within a small distance of the centerline of the track 34 being accessed. If the head assembly 18 deviates to either side of the center of the track, mistakes can occur in writing or reading information to and from the storage medium 12.
To determine the position of the head assembly 18 on the storage medium 12, special patterns are created on the storage medium 12 in one or more of the sectors 36. The head assembly 18 is used for reading the special patterns encoded on the sectors 36 of the storage medium 12 and for generating a signal that is indicative of the location of the head assembly 18 relative to the track 34 as well as the actual track number. This signal is called a position error signal (PES). Using a servo loop based on the PES, the voice coil motor 40 positions the head assembly 18 closer to the centerline of the track 34 being accessed. Accordingly, the sectors 36 are often referred to as xe2x80x9cservo sectors.xe2x80x9d
FIG. 2 generally illustrates a disk drive system 10 including a storage medium 12 and a head assembly 18 for writing and reading information to and from the storage medium 12. The head assembly 18 includes a write head 20 for writing the information to the storage medium 12 and a read head 22 for reading information from the storage medium 12. As discussed hereinbefore, in order to maximize the accuracy with which the head assembly 18 writes and reads information to and from the storage medium 12, the position of the head assembly 18, and in particular the read head 22, should be controlled with the highest achievable accuracy in order to maximize storage density and accuracy of writing and reading data to and from the disk. The control information that is provided on the servo sectors 36 of the storage medium 12, as discussed hereinbefore, is dedicated for determining the position of the head assembly 18, the read head 22 or the write head 20.
The storage medium 12 typically is a magnetic disk comprising two layers: a magnetizable recording layer 14 and a substrate layer 16. Information is written or recorded on the storage medium 12 by magnetizing small regions within the recording layer 14 generally referred to as magnetic domains 28. As part of a write operation, as the storage medium 12 rotates in a given direction 30, the write head 20 is used to create the magnetic domains 28 on the recording layer 14, thereby creating a number of flux changes within the recording layer 14. In contrast, during a read operation, the read head 22 detects the magnetic domains 28. The total number of magnetic domains 28 that can be accommodated on a disk is indicative of the storage capacity or bit density of the storage medium 12.
The write head 20 is generally a thin film inductive head, which generally includes a coil 19 and a magnet 21 having a small gap 23. The magnet 21 is formed of a soft magnetic layer and the coil 19 is wrapped around one of the pole pieces of the magnet 21, such that when an electric current 24 is passed through the coil 19, a magnetic field 25 is created across the gap 23. The portion of the magnetic field 25 that fringes out from the gap 23 magnetizes the magnetizable recording layer 14 and thus creates the magnetic domains 28. The direction of the current 24 flowing through the coil 19 determines the polarity of each of the magnetic domains 28. Current flowing in one direction forms a magnetic domain 28 having a polarity representative of logic xe2x80x9cone.xe2x80x9d Conversely, current 24 flowing in an opposite direction forms a magnetic domain 28 having an opposite polarity representative of logic xe2x80x9czero.xe2x80x9d The magnetic domains 28 form boundary regions when they are written in a contiguous pattern. The boundary regions are detected using the read head 22.
The read head 22 of the head assembly 18 generally senses the magnetic domains 28 created on the recording layer 14 of the storage medium 12 and produces an electrical signal in response thereto. In one example, the read head 22 can be a magneto resistive head, which operates on the principle of the magneto resistive effect or the giant magneto resistive effect. In its simplest form, a magneto resistive element undergoes a change in its internal resistance when it is aligned with the flux lines of a magnetic field. If a constant electrical current is provided as an input to a magneto resistive element, a change in its internal resistance will create a corresponding change in output voltage.
At the transition of the contiguous magnetic domains 28 there exist magnetic flux fields. When the read head 22 passes over a boundary region it senses the flux field present at the transition between two contiguous magnetic domains. The read head 22 responds to the magnetic flux by producing an output signal 26 (e.g., a read-back signal) corresponding to the encoded signal written to that portion of the storage medium 12. Accordingly, as the storage medium 12 moves relative to the read head 22, the read head 22 produces a series of output signals 26 representative of the information recorded on the storage medium 12. The disk drive system 10 therefore typically includes signal processing electronic circuits, such as a servo demodulator, a disk drive controller and a servo controller, for decoding the information.
FIG. 3 illustrates generally a conventional schematic representation of information encoded on a track 34 of a storage medium. As discussed previously, the track 34 may include a data portion 42 and a servo sector 36. As discussed previously, the servo sector 36 typically includes information encoded thereon, which is used to control the position of the read head 22 relative to the track 34. Those skilled in the art will appreciate that the information written to the servo sector 36 by the disk manufacturer must never be corrupted. Otherwise, it will be virtually impossible to determine the position of the read head 22 and, hence, it will be difficult to read the information stored on the storage medium 12.
The servo sector 36 typically includes a number of sub-portions, each including specially encoded patterns, which the read head 22 encounters in turn as it moves along the track 34. Generally the first special pattern in the servo sector 36 is a write recovery pattern 44. The second special pattern is a track identification pattern 46. The track identification pattern 46 merely provides information about which track 34 the read head 22 is located on, but does not provide information about the read head""s 22 relative position within the track 34. Thus, the track identification pattern 46 alone is insufficient to determine the position of the read head 22 because the read head 22 is commonly somewhat narrower than the width of the track. Therefore, the read head 22 may be located outside of the optimum position while it is still within the correct track 34. The third special pattern is a servo burst pattern 48, which contains the information required to determine the exact position of the read head 22 within the track 34. The servo burst pattern 48 in a general sense is the subject of the present invention.
Finally, there are other special patterns 50 that contain information for calibration, automatic gain control (AGC) and the like. Automatic gain control can be used, for example, when the read head 22 flies closer than desired to the surface of the storage medium 12 and thereby generates relatively higher amplitude signals than if the read head 22 was flying at a normal distance away from the surface of the storage medium 12. Accordingly, the AGC electronically and automatically adjusts the read head 22 signal amplitude when the read head 22 is flying at a distance from the surface of the storage medium 12, which is other than a predetermined distance. Although the actual information written to the servo sector 36 can vary from manufacturer to manufacturer, the information provided in the servo sector 36 described herein is typical of what is included in a servo sector 36.
As discussed hereinbefore, the invention generally relates to the servo burst pattern 48 of the servo sector 36. The reason for encoding servo burst patterns 48 in the servo sector 36 of a storage medium 12 is to keep the head assembly 18 properly oriented relative to a track 34 of the storage medium 12 during a xe2x80x9cwritexe2x80x9d or a xe2x80x9creadxe2x80x9d operation. In operation, because the position of the head assembly 18 must be continuously monitored and adjusted, the head assembly 18 can move out of alignment or drift as a result of many reasons. The most prevalent reason for moving out of alignment is due to vibration and shock of the disk drive assembly 32. For example, the storage medium 12 can be subjected to vibration resulting from bumping the disk drive system or operating it in a vibratory environment (e.g., on a plane or in a car). Also, in operation the storage medium 12 generally spins very fast, thereby creating air turbulence that tends to push the head assembly 18 out of alignment. All of these factors tend to cause the head assembly 18 to move out of alignment during a write or a read operation. It is therefore desirable to constantly monitor the position of the head assembly 18 and make the necessary adjustments to keep it centered within the current data track 34 as accurately as possible. Therefore, to properly control the position of the head assembly 18 and adjust it to a desired position, the location of the head assembly 18 relative to the storage medium and how far it has drifted from a desired location must be known.
Accordingly, information is encoded in the form of servo burst patterns 48 on the storage medium 12 to provide the disk drive assembly 32 control electronics with a position error signal (PES). The control electronics use the position error signal to adjust the position of the head assembly 18 to a desired predetermined position on the storage medium 12. Generally, the desired predetermined position of the head assembly 18 is along the center of a track 34.
Disk manufacturers typically set aside the servo sectors 36 on the storage medium 12 where the special patterns can be written. The special patterns provide a read-back signal, which is demodulated to produce the position error signal. A storage medium 12 typically includes fifty to two hundred servo sectors 36. The servo sectors 36 are generally separate from the data portions on the tracks 34 and occupy about 10-20% of the entire storage medium 12 surface area. Those skilled in the art will appreciate that the total amount of disk area that a manufacturer dedicates to servo sectors 36 is a compromise between the need for accuracy in determining the position the head assembly 18 and the need for maximizing the disk""s storage capacity.
The process of writing the special patterns comprising the servo sectors 36 must generally be conducted under controlled conditions in order to keep the storage medium 12 from vibrating during the writing process. The special patterns are carefully written to the servo sectors 36 one track at a time. Generally, there is no data written in the data portions of the tracks 34 during this operation. The process of writing the special patterns to the servo sectors 36 takes a long time to complete, e.g., from several minutes up to one half hour per storage medium 12. As a result, writing servo patterns can be the single most time consuming phase of the disk drive assembly process, and for that reason it is a very costly process in terms of time and in terms of the equipment required to perform the writing operation. For example, the machine that is required to perform this operation is very expensive. Finally, the procedure is costly because it must be performed in a pristine environment such as inside a clean room where the disk drive is assembled. Furthermore, there are equipment maintenance costs to consider, and the equipment takes up valuable space in the clean room. Accordingly, there is a need in the disk drive manufacturing art for a servo pattern that can be written faster, more accurately and less expensively.
Disk drive manufacturers must be very precise about writing and encoding information to the storage medium 12. On the other hand, reading information from the disk can be done somewhat less precisely. This is true because the information can always be read over again or an algorithm can be executed to determine the accuracy of the information. If the read head 22 does not read the information properly, the disk drive system can merely wait until that portion of the storage medium 12 rotates around to read the information again. However, if the information is incorrectly written to the storage medium 12, permanent errors and mistakes will be encoded on the disk and errors in the operation of the disk drive system will result. Therefore, disk manufacturers must be very careful, and must precisely write and encode information to the storage medium 12.
Prior art special pattern types include amplitude patterns, null patterns, time-of-flight patterns and dual-frequency (1F/2F) patterns. In amplitude modulation patterns, the information is encoded in the amplitude of the PES signal. In addition, in a phase modulation pattern, the information is encoded in the phase of the PES signal. Various methods for encoding information on a storage medium 12 using amplitude modulation and phase modulation are well known. There also exist methods that utilize the relevant amplitudes of two discrete frequencies to encode information, e.g., the dual frequency (1F/2F) pattern. Related art servo burst patterns 48 generally make a trade-off between accuracy of position, storage density optimization, ease of manufacture, and the like. Following is a description of related art servo burst patterns provided within the servo sectors 36.
FIG. 4 illustrates a servo burst pattern 44, commonly referred to as a split-burst amplitude pattern, within the servo sector 48 of the storage medium 12. The storage medium 12 includes one or more tracks of which only three are shown 35A, 35B and 35C. The split-burst amplitude pattern 44 comprises two portions, a first portion 44A and a second portion 44B, and can be written to the servo sector 36 using the write head 20 of the head assembly 18, for example.
The ideal position for the read head 22 as it traverses one of the tracks 35A-C is typically along the central line of the track (line 42 of the track 35B, for example, as illustrated in FIG. 4). The width of the tracks 35A-C is generally greater than the width of the read head 22. In addition, the servo burst regions 44A and 44B are not positioned wholly within the track 35B, but rather they straddle the adjacent tracks 35A and 35C. The principle of operation of the split-burst pattern 44 is based on the relative magnitude of magnetic signal picked up by the read head 22 from the individual split burst patterns 44A and 44B. For example, one half of the split burst pattern 44A is positioned on track 35B and the other half is positioned on track 35A. Similarly, one half of the split burst pattern 44B is positioned on track 35B while the other half is positioned on track 35C. If the read head 22 is centered within the track 35B, it will detect an equal amount of magnetic flux from both burst patterns 44A and 44B. In other words, the read head 22 will detect an equal amount of signal amplitude from the first burst pattern 44A as it does from the other burst pattern 44B, resulting in zero difference seen by the read head 22.
As can be seen from the illustration, as the read head 22 drifts away from the line 42 (due to vibration, mechanical shock, etc.) it can move to a new position 22A, for example. (Throughout this description the position of the shifted read head 22 will be illustrated as 22A although it is always the same read head 22.) Once the read head 22A has drifted from its centered position along the track 35B, one end of the read head 22A will detect different amounts of magnetic flux from the first burst pattern 44A than it will detect from the second burst pattern 44B. For example, as the read head 22A shifts toward track 35A, it will detect a greater amount of magnetic flux from the 44A portion of the burst pattern than it will detect from the 44B portion of the burst pattern. Thus, the resulting output signal, which is a measure of the relative difference in magnitude of magnetic flux detected by the read head 22A as it passes over sequentially over burst patterns 44A and 44B, will be some value other than zero.
The disk drive controller monitors the amplitude of the signal detected by the read head 22 as it passes over the first burst pattern 44A and compares it to the amplitude of the signal detected when the read head 22 passes over the second burst pattern 44B. Accordingly, if the read head 22 is exactly in the center of the track 35B, then half of the read head 22 traverses the 44A portion and half of the read head 22 traverses the 44B portion of the servo burst pattern 44A-B. Accordingly, the detected signal amplitude will be fifty percent of maximum for the 44A portion, and likewise, as the read head 22 travels into the 44B portion, the detected signal amplitude will be half of the total signal and the amplitude will also be at fifty percent of maximum. The disk drive controller compares the difference between the two amplitudes. Accordingly, if signals of equal amplitude are detected from the 44A and 44B portions of the servo burst pattern 44, the total error signal generated by the disk drive controller will be zero. This indicates that the read head 22 is on center and no correction is required.
On the other hand, if the read head 22A were to be displaced toward track 35A, for example, it may detect 75 percent of the maximum signal amplitude from the first portion 44A of the burst pattern and 25 percent of the maximum signal amplitude from the 44B portion of the burst pattern. Accordingly, under the given scenario, the disk drive controller will generate a 50 percent position error signal. Likewise, if the read head 22A moves in the direction of track 35C by a similar amount, the disk drive controller will detect 25 percent of the signal amplitude from the first portion 44A of the burst pattern and 75 percent of the signal amplitude from the second portion 44B of the burst pattern. In this scenario, the disk drive controller will generate a position error signal of the same magnitude except having an opposite polarity. Once the disk drive controller decodes the position error signal, it provides a position error correction signal to the coil motor 40 in order to physically reposition the read head 22A to its centerline position 42. The split-burst pattern 44 is a commonly used pattern because it is the least expensive to make and can be easily made with current manufacturing techniques using write heads 22 having a rectangular geometry.
Another known servo pattern is the so-called xe2x80x9cdual frequency 1F/2F pattern,xe2x80x9d illustrated in FIG. 5. The dual-frequency 1F/2F servo burst pattern 48A-B includes a first pattern 48A (1F), which generates a signal at a first frequency, and a second pattern 48B (2F), which generates a signal at a second, higher, frequency. The frequency of the signal produced by the second pattern 48B is generally double the frequency of the signal that is produced by the first pattern 48A.
As discussed above, one method of operating the disk drive assembly 32 is by positioning the read head 22 such that it traverses any of the tracks 35A-C along their centerline. Accordingly, if the read head 22 is traversing track 35B, it should be centered along the line 42 of the track 35B. As the read head 22 travels along the line 42, it simultaneously detects the super position of the signals generated by the 1F pattern 48A and the 2F pattern 48B. The disk drive controller then filters the super imposed signal comprising the 1F frequency component and the 2F frequency component and compares the relative amplitudes of the 1F signal component to the 2F signal component.
If the read head 22 is traversing the data track 35B along line 42, the disk drive controller detects equal values of amplitude (e.g., equal contributions from the signal at 1F as the signal at 2F) and produces an error correction signal of zero. However, if the read head 22A is displaced towards either track 35A or 35C, then the super imposed resulting signal will be different in that it will have more amplitude contribution from one of the two frequency signals and less amplitude contribution from the other. The disk drive controller will filter and detect the relative difference in amplitudes, and provide an error correction signal of the appropriate polarity to the coil motor 40.
Despite its usefulness, however, the dual-frequency 1F/2F servo burst pattern 48 has several shortcomings. First, it is somewhat more difficult to create this type of a pattern versus the split-burst servo pattern 44. Second, the read head 22 may respond in a different way to different frequencies. For example, the frequency response of the read head 22 at one frequency, e.g., one megahertz, may produce a signal of given amplitude. But, when the frequency is doubled, e.g., two megahertz, the frequency response of the read head 22 may produce a signal of different amplitude. To balance out the varying frequency response, both the read head 22 and the disk drive electronics will have to be calibrated, thus adding to the expense of this technique.
Because calibration can be difficult and because the 1F/2F servo burst patterns 48 can be somewhat more difficult to produce than the split-burst servo pattern 44, the dual-frequency 1F/2F servo burst pattern 48 is not as popular. Furthermore, a disadvantage common to both the split-burst servo pattern 44 and the dual-frequency 1F/2F servo burst pattern 48 is that each of these patterns works only on a single track width. As the track width narrows the format efficiency drops, making it tougher to write accurately defined patterns.
Another related art servo pattern used to generate position error signals is a phase pattern. Unlike the amplitude based servo burst patterns 44, 48 described above, the phase pattern is not limited to a single track, but rather crosses several tracks. The feature of being able to operate across several tracks is important because as track widths get narrower, it becomes harder to create well-defined servo patterns within a single track. Accordingly, the phase pattern increases the formatting efficiency and makes it easier in some ways to create larger servo patterns that provide sufficiently detailed information about the location of the read head within a particular track.
A typical phase servo pattern 54A-B is illustrated in FIG. 6. As shown in FIG. 6, the phase servo pattern 54A-B has a chevron shape that spans across several tracks 35A-C and includes two portions, a first portion 54A and a second portion 54B.
The operation of the phase servo pattern 54A-B is as follows. As the read head 22 moves along the line 42 of a track 35C (traversing the magnetic domain transitions 57 that form the chevron phase servo pattern 54A-B), a first read-back signal 62 having a first frequency is generated. A pulse in the first read-back signal 62 is generated at every magnetic domain transition 57 traversed by the read head 22 as it moves along the line 42 of the track 35C from left to right.
When the position of the read head 22A is displaced either up or down, a second read-back signal 64 is generated by the magnetic domain transitions 57. The second signal 64 will have the same frequency as the first read-back signal 62, but the pulses occur at a certain phase difference 66 that is proportional to how far the read head 22A has drifted away from the line 42. Accordingly, by measuring the phase difference 66 between the first and second read-back signals 62, 64, the disk drive controller can determine how much position error signal to generate in order to restore the position of the read head 22A to the centerline position of the track.
A problem with the chevron phase servo pattern 54A-B is that it is more difficult to create on a disk than the amplitude servo patterns 44, 48 discussed hereinbefore. The phase servo pattern 54A-B is difficult to create because the shape of the write head 20 used to produce the pattern on the disk is rectangular, thus making it difficult to produce the chevron shape 54A-B. The write head 20 is generally orthogonal to the line 42, whereas the chevron shape is ideally canted in relation to the line 42. It is also very time consuming because to obtain a uniform edge along the boundary of the chevron pattern 54A-B, the edges of the rectangular write head have to line up precisely. On the other hand, the amplitude servo burst patterns 44, 48 do not require that the edges line tip precisely as long as the amplitude of the output signal is correct.
In view of the drawbacks noted before for each of these servo pattern types, there exists a need in the disk drive art for an accurate servo pattern that is easier to encode and that spans several tracks on a storage medium. There is also a need for a servo pattern that is less noisy and that has a more linear response than existing patterns.
According to one embodiment, the present invention is directed to a method of encoding a storage medium. The method includes forming a frequency modulation servo pattern on the storage medium. The frequency modulation servo pattern may be, for example, an absolute frequency modulation servo pattern or a differential frequency modulation servo pattern. According to one embodiment, forming the frequency modulation pattern may include forming a pattern comprising a plurality of multi-sided elements, each element having a first end and a second end, such that a distance between sides of adjacent elements are spaced farther apart at the first end than at the second end. Consequently, the pattern may provide a read-back signal having a continuously varying frequency according to the position of a read head relative to the storage medium.
According to another embodiment, the present invention is directed to a disk drive. The disk drive includes a storage medium including a frequency modulation servo pattern encoded thereon, a head for reading the frequency modulation servo pattern and for producing a read-back signal therefrom, a servo demodulator in communication with the head for receiving the read-back signal and producing a position error signal therefrom, and a servo coupled to the head for moving the head relative to the surface of the storage medium in response to the position error signal.
In contrast to prior disk drive servo techniques, the present invention provides a frequency modulation servo pattern that provides accurate position error data that is easy to encode on the storage medium. In addition, the servo pattern of the present invention may be encoded to span several tracks on the storage medium. Moreover, the servo pattern of the present invention may be less noisy and may have a more linear response than existing patterns. These and other inventions will be apparent from the detailed description hereinafter.